According to this search article at ZDNET,
The worst search experiences are in using internal site searches rather than the mega search engines. Usability guru and Nielsen Norman Group principal Jakob Nielsen characterized internal, intranet search implementations as "beneath contempt." Many users avoid corporate sanctioned, internal search engines, and use Web search engines to find information outside the firewall related to their company.
This is a very interesting statement, although I don't completely agree with it. Depending on the site, sometimes a simple search solution is all that is really needed.